Polarity reversal protection circuit for a final electronic power stage

ABSTRACT

A polarity reversal protective circuit for an electronic power output stage that can be driven via a drive circuit is described. In the case of an incorrect polarization of a supply voltage, power FETs are used in at least one supply terminal to interrupt a connection of the wrongly applied supply voltage to the drive circuit and the power output stage. A definite polarity reversal protection is obtained using a circuit that is improved with respect to cost and space requirement. A single N-channel power FET is looped, with its source-drain path, into the positive supply terminal of the supply voltage. The source terminal of the N-channel power FET is connected with the positive supply terminal. The gate terminal of the N-channel power FET is connected to a collector of a PNP transistor, which is additionally connected to an output of the drive circuit via a current-limiting resistor. An emitter of the PNP transistor is connected—via a diode that is conductive when the supply voltage is wrongly applied—to the positive supply terminal. The base of the PNP transistor is connected—via a base resistor—to a negative (ground) supply terminal of the supply voltage.

FIELD OF THE INVENTION

The present invention relates to a polarity reversal protective circuitfor an electronic power output stage that can be driven via a drivecircuit, in which, in case of incorrect polarization of a supplyvoltage, power FETs are used in at least one supply terminal tointerrupt a connection of the incorrectly applied supply voltage to thedrive circuit and the power output stage.

BACKGROUND INFORMATION

A polarity reversal protective circuit of this sort is necessary becausethe electronic components of the drive intergrated circuit (IC) with thecharge pump, and the power circuit position switch, can be destroyed ifthe polarization of the supply voltage is wrong.

It is conventional to loop a diode into a supply terminal of the supplyvoltage. This diode must be matched to the power of the power circuitposition switch, and causes an additional power loss. If the supplyvoltage is applied with the wrong polarity, the diode receives currentin the blocking direction, and thus prevents a flow of current via thedrive IC and the power output stage.

The series circuit of a diode and the winding of a switch-off relay hasalso been connected between the supply terminals. In this context, thediode is looped in so that it becomes conductive when the polarity ofthe supply voltage is incorrect, so that the operating current for therelay can flow. The connection to the drive IC and to the power outputstage is interrupted via relay contacts. A disadvantage of this polarityreversal protective circuit is the space requirement and the vibrationsensitivity of the relay. This is particularly unacceptable for aspace-saving construction of an electronic power output stage forsupplying loads in motor vehicles.

In a conventional electronic polarity reversal protective circuit, inthe positive supply terminal of the supply voltage, two N-channel powerFETs are connected in series with their source-drain paths, the sourceterminals being connected with one another. The one drain terminal isconnected to the positive supply terminal, and the other drain terminalleads to the drive IC and the power output stage. The gate terminals ofthe two N-channel power FETs are connected in parallel, and are drivenby the output of a charge pump. This polarity reversal protectivecircuit requires two expensive N-channel power FETs, and still has twicethe power loss in the direction of connection. However, given incorrectpolarization of the supply voltage one of the N-channel power FETs isalways blocked in order to protect the drive circuit and the poweroutput stage.

SUMMARY OF THE INVENTION

An object of the present invention a polarity reversal protectivecircuit of the type mentioned above having few space-saving elements, sothat it is economically advantageous and guarantees clear polarityreversal protection with a low power loss.

This object is achieved in that a single N-channel power FET is looped,with its source-drain path, into the positive supply terminal of thesupply voltage the source terminal (S) being connected with the positivesupply terminal, and in that the gate terminal of the N-channel powerFET is connected with the collector of a NPN transistor, which isadditionally connected to the output of the drive circuit via acurrent-limiting resistor, and in that the emitter of the NPN transistoris connected—via a diode that is conductive when the supply voltage isincorrectly applied—to the positive supply terminal, and the base of theNPN transistor is connected—via a base resistor—to the negative (ground)supply terminal of the supply voltage.

This polarity reversal protective circuit requires only a singleN-channel power FET, which trips fully when the supply voltage iscorrectly applied and has a very low power loss. The second, expensiveN-channel power FET is replaced with the NPN transistor, the diode andthe base resistor, i.e. with three small-signal components, and when thepolarization of the supply voltage is incorrect a definite blocking ofthe N-channel power FET looped into the positive supply terminal isachieved. In relation to the conventional polarity reversal protectivecircuit having two N-channel power FETs, this is advantageous not onlyin terms of cost and space requirement (with respect to surface), butalso from a technical point of view, e.g. lower voltage drop.

According to another embodiment, the drive circuit is fashioned with acharge pump as an IC.

BRIEF DESCRIPTION OF THE DRAWINGS

The Figure shows a polarity reversal protective circuit according to thepresent invention.

DETAILED DESCRIPTION

Supply voltage Vs is connected with the positive pole to positive supplyterminal + and with the negative (ground) pole to negative supplyterminal −. An N-channel power FET T2 is looped, with its source-drainpath S-D, into positive supply. Source Terminal S is connected withpositive supply terminal +. Drive circuit AST-IC and power output stageLEST is disconnected at drain terminal D. Negative supply terminal − isswitched through smoothly and leads directly to drive circuit AST-IC andto power output stage LEST, which, according to the power requirement,can be constructed, for example, as a simple switching stage, ahalf-bridge circuit or a full-bridge circuit.

N-channel power FET T2 must be matched, in the conducting direction, tothe required power of power output stage LEST. The driving of N-channelpower FET T2 is sufficient, since this produces only low power loss inthe conducting direction.

If, however, supply voltage Vs is applied to the supply terminals withthe wrong polarity, then the driving of N-channel power FET T2 does nottake place. If drive circuit AST-IC is fashioned with a charge pump LPas an IC, then output A of drive circuit AST-IC is connected with thesupply lines via parasitic diodes PD1 and PD2.

In order to prevent the flow of current in the reverse direction,N-channel power FET T2 is blocked when supply voltage Vs is wronglyapplied. For this purpose, a NPN transistor T1 is provided that becomesconductive and that applies gate terminal G of N-channel power FET T2 tosource via diode D1, so that N-channel power FET T2 is switched off.

Emitter E of NPN transistor T1 is connected with the negative potentialat positive supply terminal + via diode D1, which is conductive in thisdirection, so that NPN transistor T1 becomes conductive and applies gateterminal G of N-channel power FET T2 to source via diode D1. Current canthus no longer flow in the opposite direction to drive circuit AST-ICand power output stage LEST via N-channel power FET T2, so that adefinite polarity reversal protection is provided. Diode D1 is alsorequired, because the allowable negative blocking voltage of NPNtransistor T1 from base B to emitter E is mostly smaller than requiredsupply voltage Vs that is applied.

Without diode D1, NPN transistor T1 and base resistor R1, parasiticdiode PD2 would become conductive to ground, and would drive gateterminal G of N-channel power FET T2 conductive, so that the polarityreversal protective function would not be provided. This flaw is removedby the introduction of these three small-signal components. It is truethat the omission of the second N-channel power FET, which wouldotherwise be required, is achieved at the cost of introducing thesethree elements. However, this is preferable in terms of cost and spacerequirement, as well as technically.

What is claimed is:
 1. A polarity reversal protective circuit for anelectronic output stage drivable by a drive circuit, power FETs beingused in at least one supply terminal to interrupt a connection of awrongly applied supply voltage, comprising: a n-channel power FET havinga source terminal, a gate terminal and drain terminal, the N-channelpower FET being looped with a source-drain path of the N-channel powerFET into a positive supply terminal of a supply voltage, the sourceterminal being coupled to the positive supply terminal; and a NPNtransistor having a collector, a base, and an emitter, the collectorbeing coupled to the gate terminal of the N-channel power FET, thecollector being further coupled via a current-limiting resistor to anoutput of a drive circuit, the emitter being coupled via a diode to thepositive supply terminal of the supply voltage, the diode beingconductive when the supply voltage is incorrectly applied, the basebeing coupled via a base resistor to a negative supply terminal of thesupply voltage.
 2. The circuit according to claim 1, wherein: the drivecircuit is an integrated circuit having a charge pump.